The effects of CNS treatment (cranial irradiation and intrathecal chemotherapy) on the cognitive and perceptual functioning of children with leukemia are investigated. One study compared a group of leukemic children in remission with a matched group of sibs on measures from a battery of pyschological tests (Wechsler, Bender-Gestalt). These groups are follow-up 12-15 months after the initial assessment. Non-CNS treated comparison groups are also being assessed using these procedures to control for effects other than CNS treatment. A second study deals with another sample studied prospectively. In this instance children in remission from leukemia are evaluated on the same battery of psychological tests as in the first study. They are tested just prior to prophylactic CNS treatment and are re-evaluated one year later to determine if any possible brain changes brought on by the CNS treatment have resulted in learning disabilities and diminished intellectual functioning. A five year longitudinal study investigating the neuropsychological correlates of CNS treatment of over 180 leukemic cases at various Children's Hospitals is being initiated.